1. Field of the Invention
The present invention relates to an automatic transmission for a vehicle, particularly, for a motorcar of the type including a main transmission and an auxiliary transmission.
2. Description of the Prior Art
A hitherto known four wheel driving automatic transmission of the type including a main transmission and an auxiliary transmission has a four wheel driving transfer attached thereto which is so constructed that speed changing such as two wheel driving in the directly connected state (H2), four wheel driving in the directly connected state (H4), four wheel driving in the speed reduced state (L4) or the like is controlled by selectively actuating a manual shift. The four wheel driving vehicle with the above-described four wheel driving automatic transmission mounted thereon usually uses L4 having a high gear torque on a steeply inclined road, rugged ground or the like under severe running conditions.
However, it has been found that the conventional automatic transmission as constructed in the above-described manner fails to effectively function when it assumes the low gear torque range at a time when the high speed range (overdrive) is also selected or operating. Further, when overdrive and the L4 range are simultaneously engaged, the planetary gears used for overdrive are caused to rotate at a high rotational speed. This has a substantial detrimental effect on the operational life of gears and associated components in the transmission.